Patent GB-647585-A (GB)
Title: Improvements relating to control systems for aircraft power plant installations
IPC: B64C11/40
Filed: 1947-05-13
Abstract: 647,585. Gas turbine plant. MORDELL, D. L., and HOLLIDAY, J. B. May 13, 1947, No. 12939. [Class 110(iii)] [Also in Groups XXIX and XXXIII] A control system for an aircraft power plant having a gas-turbine-engine driving a variable-pitch airscrew or a variablepitch ducted fan adjustable to maintain a preselected engine speed has control lever means for preselecting such speed and a pressure responsive device to control the delivery of fuel to the engine, this device accordance with a desired function of the instantaneous engine speed and to a load dependent on the actual fuel flow to the engine. As shown the fuel supply to the combustion chambers 11, Fig. 1, of a gas turbine engine comprising a turbine 15 driving a compressor 10 and an airscrew 17 is controlled by an engine driven multiplunger pump 14 the swashplate 31 of which is adjusted by a lever 32. The pitch of the airscrew is adjusted by a servomotor controlled by a valve 26 actuated by a governor 24. The governor 24 and a pump 28 supplying the servomotor are driven from the airscrew. The governor loading is adjusted to vary the datum speed by a lever 30 geared to the abutment 29. The pump control lever 32 is adjusted by a servo-piston 33, Fig. 2, under the control of a piston valve 35 actuated by the pressure responsive device (which is shown as a diaphragm 36) and supplied from an engine driven pump 41 through pipes 42, 49. The diaphragm 36 is loaded by a spring 37 of which the abutment 38 is adjusted by the piston 33. The spring loading of the diaphragm therefore varies with the position of the piston 33 and therefore' with the actual flow of fuel to the engine. The diaphragm 36 is also subject to the pressure difference across an orifice 55 adjusted by a barometric capsule 58 in accordance with the ambient or engine air intake pressure. The orifice 55 is supplied by a pipe 53 from a chamber on the underside of a spring- loaded piston 43 which has a port 51 in its skirt co-operating with a port 52 leading to the pipe 53. The upper side of the piston 43 is supplied from the pump 41 so that the position of the piston and hence the relation of the ports 51, 52 is related to the engine speed. The two sides of the piston 43 are connected through a fixed restriction 46. A relief valve 48 maintains a substantially constant pressure in the lower piston chamber so that the flow in the pipe 53 is related to engine speed. The pipe 53 which leads eventually to a sump 40 contains a second orifice 56 this being adjusted by a valve 60 actuated by a diaphragm 61 subject through pipes 62, 63 to the pressure drop across the ports 51, 52 and loaded through a lever 64 by a spring 65 adjusted by a thermally responsive device 66 in accordance with the ambient or engine air intake temperature. The pressure drop across the diaphragm 36 and therefore the fuel supply is thus dependent on the ambient air temperature and pressure, and on the instantaneous engine speed. In a modified arrangement, Fig. 3 (not shown) the loading spring 37 is dispensed with and the two ends of the piston valve 35 are subject to the pressure drop across a restricted orifice in the fuel supply pipe 13, Fig. 1, or in the suction pipe 21. Fig. 4 shows a fuel control arrangement similar to Fig. 2 but modified so that pressure differences corresponding to different functions of engine speed may be selectively imposed on the diaphragm 36. Thus the skirt of the piston 43 is provided with three different ports 51a, 51b, 51c co-operating with ports 52a, 52b, 52c respectively. The ports 51c, 52c are permanently connected to the pipe 53 but the ports 51b, 52b and 51a, 52a can be cut off in succession from the pipe 53 by a valve 80 operated by a pilot's control lever 81, Fig. 5. Normally the valve 80 is in the position shown and the ports 51b, 52b and 51c, 52c are effective to create a pressure appropriate for steady running. If the lever 81 is moved to the right all the ports 51, 52 are effective and the pressure developed in the pipe 53 will establish a fuel flow appropriate for maximum torque during acceleration. Similarly if the lever 81 is moved to the left, the ports 51c, 52c only are effective and the fuel flow is that appropriate for deceleration. The valve 80 is operated in conjunction with the speed datum controlling lever 30 by the lever 81 as described in Specification 647,584. Thus the valve 80 is normally maintained in the position shown by centralizing springs 97 and is coupled to the lever 81 by a floating lever 94 pivoted on a rod 91 connected to a dashpot piston 92 the opposite sides of which are in communication through a byepass controlled by a needle valve 98. The valve 98 is adjusted according to the ambient atmospheric pressure by a capsule 99. The rod 91 is connected through a further floating lever 88 to the control valve 87 of a servo-piston 82 which adjusts the speed datum varying lever 30. The piston rod 84 carries a cam 85 which acts through a lever 90 and the floating lever 88 to return the valve 87 to its neutral position. The contour of the cam 85 is shaped so that the rate of travel of the dashpot piston 92 is correlated with that of the servo-piston 82 and so that the rate of change of airscrew pitch dependent on the instantaneous setting of the variable datum of the airscrew pitch governor. Initial movement of the lever 81 actuates the valve 80 and loads the dashpot. The valve 80 gradually returns to its normal position under the control of the dashpot and simultaneously the speed datum lever 30 and therefore the airscrew pitch are adjusted at a rate controlled by the dashpot and determined by the ambient air pressure and remain so adjusted according to the position of the lever 81. An electrically-actuated constant speed unit may be used. An overtravel of the lever 81 beyond the maximum speed position may cause operation of a further valve which progressively applies to the diaphragm 36 a pressure appropriate to the fuel flow for maximum power. The piston and cylinder 43, 44 may have a fourth set of ports 51, 52 which create a pressure lower than that appropriate to minimum power for operation on the fuel control of an engine temporarily disconnected from an airscrew to which two or more engines are normally coupled. The fuel control may act alternatively on a throttle controlling the delivery of a variable delivery pump which maintains a predetermined delivery pressure or of a constant delivery pump having a byepass. According to the Provisional Specification a pressure related to the actual fuel flow to the engine and also appropriately varied in accordance with the ambient air temperature and pressure to satisfy a given flow law is produced by the apparatus described in the Specification 647,342 and is balanced against a pressure modulated in accordance with a desired function of the instantaneous engine speed.